Between 1884 and 1888, Clark constructed a 34-room, Tiffany-decorated home on West Granite Street, incorporating the most modern inventions available, in Butte, Montana. This home is now the Copper King Mansion bed-and-breakfast and museum.[4] In 1899 Clark built the Columbia Gardens for the children of Butte. It included flower gardens, a dance pavilion, amusement park, lake, and picnic areas.

Clark served as president of both Montana state constitutional conventions in 1884 and 1889.

Political cartoon depicting William A. Clark bribing state legislators.

Clark yearned to be a statesman and used his newspaper, the Butte Miner, to push his political ambitions. At this time, Butte was one of the largest cities in the West. He became a hero in Helena, Montana, by campaigning for its selection as the state capital instead of Anaconda. This battle for the placement of the capital had subtle Irish vs. English, Catholic vs. Protestant, and Masonic vs. non-Masonic elements. Clark's long-standing dream of becoming a United States Senator resulted in scandal in 1899 when it was revealed that he bribed members of the Montana State Legislature in return for their votes. At the time, U.S. Senators were chosen by their respective state legislators. The corruption of his election contributed to the passage of the 17th Amendment. The U.S. Senate refused to seat Clark because of the 1899 bribery scheme, but a later senate campaign was successful, and he served a single term from 1901 until 1907. In responding to criticism of his bribery of the Montana legislature, Clark is reported to have said, "I never bought a man who wasn't for sale."[5]

In a 1907 essay Mark Twain portrayed Clark as the very embodiment of Gilded Age excess and corruption:

He is as rotten a human being as can be found anywhere under the flag; he is a shame to the American nation, and no one has helped to send him to the Senate who did not know that his proper place was the penitentiary, with a ball and chain on his legs. To my mind he is the most disgusting creature that the republic has produced since Tweed's time.[7]

Mark Twain's comments must be taken in context. He was a close friend of Henry H Rogers, one of the organizers of the Amalgamated Copper Company to gain control of Butte's mines in opposition to Clark. Twain is not an unbiased observer.[8]

After Kate's death in 1893, William married his second and final wife, the woman who had been his teenage ward, Anna Eugenia La Chapelle (March 10, 1878 Michigan – October 11, 1963 New York). They claimed to have been married in 1901 in France. Anna was 23 and William was 62.[11] They had two children:

Huguette (pronounced: oo-GETT), born in Paris, France in June 1906, was known as a reclusive heiress and was the youngest child of Clark with his second wife, Anna Eugenia La Chapelle. She married once, but divorced less than a year later. She led a reclusive life thereafter, communicating very little with the public including her extended family. She lived in three magnificent apartments, with a total of 42 rooms, on New York's Fifth Avenue at 72nd Street, overlooking Central Park. In 1991, she moved out of her apartment in New York and for the remainder of her life, voluntarily lived in various New York City hospitals.

In February 2010, she became the subject of a series of reports on msnbc.com after it was reported that the caretakers of her three residences (including a $24 million estate in Connecticut, a sprawling seaside estate in Santa Barbara, California and her Fifth Avenue apartments valued at $100 million) had not seen her in decades. These articles were the basis for the 2013 bestselling book Empty Mansions: The Mysterious Life of Huguette Clark and the Spending of a Great American Fortune by investigative reporter Bill Dedman.

Her final residence was Beth Israel Medical Center, where she died on the morning of May 24, 2011, age 104.[14] Huguette's extraordinary collection of arts and antiquities were consigned to go on the auction block at Christie's in June 2014, over three years after her death.[15]

Clarkdale, Arizona, named for Clark, was the site of smelting operations for Clark's mines in nearby Jerome, Arizona. The town includes the historic Clark Mansion, which sustained severe fire damage on June 25, 2010. Clarkdale is home to the Verde Canyon Railroad wilderness train ride which follows the historic route that Clark had constructed in 1911 and home to the Copper Art Museum.[16]

Clark's art collection was donated to the Corcoran Gallery in Washington, D.C. after his death, greatly enriching that museum's holdings of European as well as American art. The Clark donation also included the construction of a new wing for the Corcoran, known appropriately as the Clark Wing.